This invention relates to a heat load reduction film in an electrically heated transparency such as may be employed in a vehicle to provide defrosting, deicing, or defogging capability.
It is known to pass electric current through a transparent conductive coating on a transparency in order to raise the temperature of the transparency. Generally, a source of electrical potential is connected to the conductive coating by way of a pair of bus bars along opposite sides of the area of the transparency to be heated. The bus bars have low resistivity relative to the coating and are intended to distribute the current evently over the area to be heated. The bus bars may be comprised of metallic foil strips, but in the case of glass transparencies they preferably are comprised of a metallic-ceramic frit material fused onto a surface of the transparency.
A typical arrangement includes bus bars configured as substantially parallel stripes on opposite sides of the heated area, with electrical leads attached to each bus bar and extending away from the opposite edges of the transparency as shown in U.S. Pat. Nos. 4,323,726 to Criss et al and 4,668,270 to Ramus. Locating the leads on the same side of the transparency and preferably closely adjacent to each other is advantageous for the sake of easier installation of the transparency in the vehicle and simplifying the connection with the electrical power source as shown in U.S. Pat. Nos. 3,895,213 to Levin and 4,543,466 to Ramus.
A preferred bus bar arrangement involves connection of the remote bus bar to the electrical circuit by way of two conductive extensions of the bus bar, each extending from opposite ends of the remote bus bar along opposite ends of the transparency as described in U.S. Ser. No. 138,008 filed Dec. 28, 1987. The conductive extensions are insulated from the conductive coating on the transparency, preferably by omitting or deleting the coating in the marginal area near the extensions.
U.S. Pat. No. 4,094,763 to Gillery et al discloses producing transparent, electroconductive articles by cathode sputtering metals such as tin and indium onto refractory substrates such as glass at a temperature above 400.degree. F. in a low pressure atmosphere containing a controlled amount of oxygen.
U.S. Pat. No. 4,113,599 to Gillery teaches a cathode sputtering technique for the reactive deposition of conductive indium oxide in which the flow rate of oxygen is adjusted to maintain a constant discharge current while the flow rate of argon is adjusted to maintain a constant pressure in the sputtering chamber.
U.S. Pat. No. 4,462,883 to Hart discloses a low emissivity coating produced by cathode sputtering a layer of silver, a small amount of metal other than silver, and an antireflection layer of metal oxide onto a transparent substrate such as glass. The antireflection layer may be tin oxide, titanium oxide, zinc oxide, indium oxide, bismuth oxide or zirconium oxide.
High transmittance, low emissivity coatings as described above generally comprise a thin metallic layer, for infrared reflectance and low emissivity, sandwiched between dielectric layers of metal oxides to reduce the visible reflectance. These multiple layer films are typically produced by cathode sputtering, especially magnetron sputtering. The metallic layer may be gold or copper, but is generally silver. The metal oxide layers described in the prior art include tin oxide, indium oxide, titanium oxide, bismuth oxide, zinc oxide, zirconium oxide and lead oxide. In some cases, these oxides incorporate small amounts of other metals, such as manganese in bismuth oxide, indium in tin oxide and vice verse, to overcome certain disadvantages such as poor durability or marginal emissivity. However, all of these metal oxides have some deficiency.
U.S. Pat. No. 4,610,771 to Gillery, the disclosure of which is incorporated herein by reference, provides a novel film composition of an oxide of a zinc-tin alloy, as well as a novel multiple-layer film of silver and zinc-tin alloy oxide layers for use as a high transmittance, low emissivity coating.
While multiple-layer, low-emissivity, high transmittance films have been made sufficiently durable for architectural applications in multiple glazed window units, such films were not sufficiently temperature-resistant to withstand high temperature processing, such as bending, tempering or laminating. Moreover, it is desirable to have a coating which serves both as a low emissivity film to reduce heat gain in an enclosed space and as a current-carrying heatable film for defogging, defrosting or deicing a transparency, particularly in a vehicle.